There are various types of interference and noise sources in a multi-carrier communication system, such as a Discrete Multiple-Tone (DMT) system. Interference and noise may corrupt the data-bearing signal on a tone as the signal travels through the communication channel and is decoded at the receiver. The transmitted data-bearing signal may be decoded erroneously by the receiver because of this signal corruption. The number of data bits or the amount of information that a tone carries may vary from tone to tone and depends on the relative power of the data-bearing signal compared to the power of the corrupting signal on that particular tone.
In order to account for potential interference on the transmission line and to guarantee a reliable communication between the transmitter and receiver, each tone of a DMT system is typically designed to carry a limited number of data bits per unit time based on the tone's Signal to Noise Ratio (SNR) using a bit-loading algorithm, which is an algorithm to determine the number of bits per tone. The number of bits that a specific tone may carry decreases as the relative strength of the corrupting signal increases, that is when the SNR is low. Thus, the SNR of a tone may be used to determine how much data should be transmitted by the tone to achieve a target bit error rate.
It is often assumed that the corrupting signal is an additive random source with Gaussian distribution and white spectrum. With this assumption, the number of data bits that each tone can carry relates directly to the SNR. However, this assumption may not be true in many practical cases where there might exist various sources of interference that do not have a white, Gaussian distribution. Impulse noise is one such noise source. Bit-loading algorithms are usually designed based on the assumption of additive, white, Gaussian noise. With such algorithms, the effects of impulse noise can be underestimated, resulting in aggressive bit loading and, consequently, an excessive rate of error.
Further, channel estimation procedures that can be designed to optimize performance in the presence of stationary impairments such as additive, white, Gaussian noise, but are often poor at estimating non-stationary or cyclo-stationary impairments, such as impulse noise. Consequently, Digital Subscriber Line (DSL) modem training procedures are typically well suited to optimizing performance in the presence of additive, white, Gaussian noise, but leave the modem receivers relatively defenseless to impulse noise.
Impulse Noise can be a difficult impairment for DSL modems. Impulse noise with duration of tens of microseconds can cause errors in all the used tones at the receiver. Further, impulse noise can have power bursts that are much higher than the background noise level causing significant performance loss. These power bursts can have a very small duty cycle, such that they do not contribute significantly to average noise power. This can result in aggressive bit loading on some or all tones in a DMT system, resulting in an excessively high bit error rate. It is thus desirable to detect the presence of and mitigate the impact of impulse noise in Asymmetric DSL (ADSL) and Very high bit-rate DSL (VDSL) and other communications systems.